1. Field of the Invention
The present invention relates to an image sensor and more particularly to an image sensor receiving a sampled signal with an increased voltage difference.
2. Description of the Related Art
In general, an image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor may have a parallel column structure as shown in FIG. 1.
FIG. 1 is a block diagram of an image sensor according to the related art.
With reference to FIG. 1, an image sensor according to the related art may include a plurality of pixels 10 disposed in a two-dimensional N×M structure, a plurality of sampling units 20-1 to 20-N with which the plurality of pixels 10 are connected in column units, a single sense amplifier 30 connected in common with the plurality of sampling units 20-1 to 20-N, and the like. Each of the sampling units 20-1 to 20-N may include a comparison portion (21-1 to 21-N), a counting portion (22-1 to 22-N) and a storage portion (23-1 to 23-N). In FIG. 1, SLp and SLn may refer to a sampled signal line pair, and doutp and doutn may refer to respective sampled signals respectively transmitted through the signal line pair SLp and SLn.
Here, when pixel signals px1 to pxN are generated and output by the plurality of pixels 10, comparison portions 21-1 to 21-N compare a voltage of the received pixel signals px1 to pxN with a reference voltage vramp, and counting portions 22-1 to 22-N count time until output results out1 to outN from the comparison portions 21-1 to 21-N are inverted or non-inverted to thus generate sampled signals and then store the generated sampled signals in the storage portions 23-1 to 23-N.
Thereafter, the storage portions 23-1 to 23-N provide sampled signal pairs (doutp, doutn) to a sense amplification unit 30 in response to addresses input thereto, and the sense amplification unit 30 senses the received signal pairs to generate data to be provided to a digital block (not shown).
However, in order to perform a sensing operation using the sense amplification unit 30, a signal of the sampled signal pair (doutp, doutn) should have a predetermined voltage level or more. For example, a voltage difference ΔV between the sampled signal doutp and the sampled signal doutn should have a predetermined voltage level or more.
However, as shown in FIG. 1, when a plurality of sampling units 20-1 to 20-N are connected in common with a single sense amplification unit 30, there may be a defect in which time taken while a signal of the sampled signal pair (doutp, doutn), for example, a voltage difference between the sampled signal doutp and the sampled signal doutn, reaches any predetermined level or more thereof, is increased in proportion to an amount of image sensor pixels. That is, as the number of image sensor pixels increases, the number of sampling units 20-1 to 20-N also increases. Therefore, the sense amplification unit 30 is connected with a relatively large number of sampling units 20-1 to 20-N, and a length of the sampled signal line pair (SLp, SLn) between the sense amplification unit 30 and the sampling units 20-1 to 20-N is also increased.
As a result, in the case of increasing the number of pixels in an image sensor, a period of time taken for a signal of the sample signal pair (doutp, doutn) to reach a signal level having a predetermined voltage level or more also increases, and thus, data readout speed of the sense amplification unit 30 may be deteriorated in proportion to the increased time. Accordingly, an image sensor according to the related art has a defect in which high speed operations cannot be supported in the case of employing a large number of pixels.